Numerous protective layers for metallic components which are intended to increase the corrosion resistance and/or oxidation resistance of the components are known from the prior art. Most of these protective layers are known under the collective name MCrAlX, where M stands for at least one of the elements selected from the group consisting of iron, cobalt and nickel, and further essential constituents are chromium, aluminum and X=yttrium, although the latter may also be partly or completely replaced by an equivalent element selected from the group consisting of scandium and the rare earth elements.
Typical coatings of this type are known from U.S. Pat. Nos. 4,005,989 and 4,034,142.
Furthermore, EP-A 0 194 392 has disclosed numerous special compositions of protective layers of the above type with admixture of further elements for various applications. In this context, the element rhenium with admixture levels of up to 10% by weight as well as numerous other elements that can optionally be added is mentioned. However, in view of the lack of more specific further ranges for possible admixtures, none of the protective layers described is qualified for special conditions, such as for example on rotor blades and guide vanes of gas turbines with high inlet temperatures which have to be operated for prolonged periods of time.
Protective layers which contain rhenium are also known from U.S. Pat. No. 5,154,885, EP-A 0 412 397, DE 694 01 260 T2 and WO 91/02108 A1. The disclosure on the action of rhenium which is revealed overall by these documents is incorporated in its entirety in the present disclosure.
EP 1 306 454 A1 likewise discloses a protective layer consisting of nickel, cobalt, chromium, aluminum, rhenium and yttrium. There are no details as to the nickel and cobalt levels.
U.S. Pat. No. 6,346,134 B1 discloses an MCrAlY layer, having a chromium content of from 20 to 35% by weight, an aluminum content of from 5 to 15% by weight, additions of hafnium, rhenium, lanthanum or tantalum as well as a high yttrium content of from 4 to 6% by weight.
U.S. Pat. No. 6,280,857 B1 discloses a high-temperature-resistant layer which includes the elements cobalt, chromium and aluminum based on nickel, with the optional addition of rhenium and necessary admixtures of yttrium and silicon.
EP 0253 754 91 provides details as to the application of a protective layer to a gas turbine component that is to be exposed to high thermal stresses.
The objective of increasing the inlet temperatures of both stationary gas turbines and aircraft engines is of considerable significance in the specialist field of gas turbines, since the inlet temperatures are important variables determining the thermodynamic efficiencies which can be achieved by gas turbines. The use of specially developed alloys as base materials for components which are to be exposed to high thermal stresses, such as guide vanes and rotor blades, and in particular the use of single-crystal superalloys, allows the use of inlet temperatures of well over 1000° C. Nowadays, the prior art permits inlet temperatures of 950° C. and above in the case of stationary gas turbines and 1100° C. and above in the case of gas turbines for aircraft engines.
Examples of the structure of a turbine blade or vane having a single-crystal substrate, which for its part may be of complex structure, are revealed by WO 91/01433 A1.
Whereas the physical load-bearing capacity of the base materials which have by now been developed for the highly stressed components does not present any major problems with a view to possible further increases in the inlet temperatures, protective layers have to be employed to achieve sufficient resistance to oxidation and corrosion. In addition to the sufficient chemical stability of a protective layer under the attacks expected from flue gases at temperatures of the order of magnitude of 1000° C., a protective layer also has to have sufficiently good mechanical properties, not least with a view to the mechanical interaction between the protective layer and the base material. In particular, the protective layer must be sufficiently ductile to enable any deformation of the base material to be followed and not to crack, since points of attack for oxidation and corrosion would be created in this way. This typically gives rise to the problem that an increase in the levels of elements such as aluminum and chromium, which can increase the resistance of a protective layer to oxidation and corrosion, leads to a deterioration in the ductility of the protective layer, which means that mechanical failure, in particular the formation of cracks, is likely under mechanical loading which usually occurs in a gas turbine. Examples of the reduction in the ductility of the protective layer brought about by the elements chromium and aluminum are known from the prior art.
WO 01/09403 A1 has disclosed a superalloy for a substrate which also contains rhenium. That document describes the fact that the intermetallic phases formed by rhenium reduce the long-term stability of the superalloy. This effect can be alleviated by the addition of ruthenium.